Circadian rhythmicity was studied by following the course of N-acetyl-transferase activity in the pineal glands of chickens in vitro. The results indicate (i) a daily change during day 1 of organ culture in constant dark that was dependent on the time the chickens were killed, (ii) equivocal persistence of the daily change in constant dark during 6 to 7 days of organ culture, (iii) an effect of light, (iv) inhibition by adrenergic agents and cycloheximide, and (v) stimulation by dibutyryl adenosine 3',5'-monophosphate and related compounds.